This application claims the benefit of Korean Patent Applications No. 2000-27850 filed on May 23, 2000, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment orientation of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed.
Liquid crystal is classified into positive liquid crystal and negative liquid crystal, depending on the electrical properties of the liquid crystal. The positive liquid crystal has a positive dielectric anisotropy such that long axes of liquid crystal molecules are aligned parallel to an electric field. Whereas, the negative liquid crystal has a negative dielectric anisotropy such that long axes of liquid crystal molecules are aligned perpendicular to an electric field.
By now, active matrix LCDs, in which the thin film transistors and the pixel electrodes are arranged in the form of a matrix, are widely used because of their high resolution and superiority in displaying moving video data.
FIG. 1 is a cross-sectional view illustrating a typical twisted nematic (TN) LCD panel. As shown in FIG. 1, the TN LCD panel has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2 includes a first transparent substrate 1a and a thin film transistor (xe2x80x9cTFTxe2x80x9d) xe2x80x9cSxe2x80x9d. The TFT xe2x80x9cSxe2x80x9d is used as a switching element to change orientation of the liquid crystal molecules. The lower substrate 2 further includes a pixel electrode 15 that applies an electric field to the liquid crystal layer 10 in accordance with signals applied by the TFT xe2x80x9cSxe2x80x9d. The upper substrate 4 has a second transparent substrate 1b, a color filter 8 on the second transparent substrate 1b, and a common electrode 14 on the color filter 8. The color filter 8 implements color for the LCD panel. The common electrode 14 serves as another electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 15 is arranged over a pixel region xe2x80x9cP,xe2x80x9d i.e., a display area. A transparent conductive material like indium tin oxide (ITO) having superior light transmittance is used for the pixel electrode 15. Further, to prevent leakage of the liquid crystal layer 10 between the lower and upper substrates 2 and 4, those substrates are sealed by a sealant 6.
As described above, because the pixel and common electrodes 15 and 14 of the conventional TN LCD panel are positioned on the lower and upper substrates 2 and 4, respectively, the electric field induced therebetween is perpendicular to the lower and upper substrates 1a and 1b. The above-mentioned liquid crystal display device has advantages of high transmittance and aperture ratio, and further, since the common electrode on the upper substrate serves as an electrical ground, the liquid crystal is protected from a static electricity.
However, the above-mentioned operation mode of the TN LCD panel has a disadvantage of a narrow viewing angle. To overcome the above-mentioned problem, an in-plane switching (IPS) LCD panel was developed. The IPS LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN or STN (super twisted nematic) LCD panel. A detailed explanation about operation modes of a typical IPS LCD panel will be provided with reference to FIGS. 2, 3A, 3B, 4A and 4B.
As shown in FIG. 2, first and second substrates 1a and 1b are spaced apart from each other, and a liquid crystal xe2x80x9cLCxe2x80x9d is interposed therebetween. The first and second substrates 1a and 1b are called an array substrate and a color filter substrate, respectively. Pixel and common electrodes 15 and 14 are disposed on the first substrate 1a. The pixel and common electrodes 15 and 14 are parallel with and spaced apart from each other. On a surface of the second substrate 1b, a color filter 25 is disposed opposing the first substrate 1a. The pixel and common electrodes 15 and 14 apply an electric field xe2x80x9cExe2x80x9d to the liquid crystal xe2x80x9cLCxe2x80x9d, then it is aligned parallel to the electric field xe2x80x9cExe2x80x9d.
FIGS. 3A and 3B conceptually illustrate xe2x80x9coff statexe2x80x9d operation modes for a typical IPS LCD device. In off state, the long axes of the LC molecules xe2x80x9cLCxe2x80x9d maintain a definite angle with respect to a line that is perpendicular to the pixel and common electrodes 15 and 14. The pixel and common electrode 15 and 14 are parallel with each other. Herein, the angle difference is 45 degrees, for example.
FIGS. 4A and 4B conceptually illustrate xe2x80x9con statexe2x80x9d operation modes for the typical IPS LCD device. In an on state, an in-plane electric field xe2x80x9cExe2x80x9d, which is parallel with the surface of the first substrate 1a, is generated between the pixel and common electrodes 15 and 14. The reason is that the pixel electrode 15 and common electrode 14 are formed together on the first substrate 1a. Then, the LC molecules xe2x80x9cLCxe2x80x9d are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the LC molecules xe2x80x9cLCxe2x80x9d are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes 15 and 14.
By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS LCD device displays images. The IPS LCD device has wide viewing angle and low color dispersion. Specifically, the viewing angle of the IPS LCD device is about 70 degrees in direction of up, down, right, and left. In addition, the fabricating processes of this IPS LCD device are simpler than other various LCD devices. However, because the pixel and common electrodes are disposed on the same plane of the lower substrate, the transmittance and aperture ratio are low. In addition, the IPS LCD device has disadvantages of a relatively slow response time and a relatively small alignment margin. Because of the small alignment margin, the IPS LCD device needs a uniform cell gap.
The IPS LCD device has the above-mentioned advantages and disadvantages. Users may or may not select an IPS LCD device depending on the intended use.
Now, with reference to FIGS. 5, and 6A to 6D, a fabricating process for a conventional IPS LCD device is provided. FIG. 5 is a plan view illustrating a unit pixel region xe2x80x9cPxe2x80x9d of a conventional IPS LCD device. As shown, a gate line 50 and a common line 54 are arranged parallel to each other, and a data line 60 is arranged perpendicular to the gate and common lines 50 and 54. Near a cross point of the gate and data lines 50 and 60, a gate electrode 52 and a source electrode 62 are disposed. The gate and source electrodes 52 and 62 integrally communicate with the gate line 50 and the data line 60, respectively. The source electrode 62 overlaps a portion of the gate electrode 52. In addition, a drain electrode 64 is disposed opposite to the source electrode 62 with an interval therebetween.
A plurality of common electrodes 54a are disposed perpendicular to the common line 54 and connected to the common electrode. The plurality of common electrode 54a are spaced apart from each other with an equal interval therebetween. A first connecting line 66 integrally communicates with the drain electrode 64, and a plurality of pixel electrodes 66a are disposed perpendicular to the first connecting line 66. First ends of the pixel electrodes 66a are connected with the first connecting line 66, and the second ends of the pixel electrodes 66a are connected with a second connecting line 68 that is disposed over the common line 54. The plurality of common electrodes 54a and the pixel electrodes 66a are spaced apart from each other and arranged in an alternating pattern. Therefore, each common electrode 54a is parallel to an adjacent pixel electrode 66a. 
FIGS. 6A to 6D illustrate a sequence of fabricating processes for an array substrate 1 of the above-mentioned IPS LCD device.
As shown in FIG. 6A, on the array substrate 1, a first metal layer is deposited and patterned to form the gate electrode 52 and the plurality of common electrodes 54a. The first metal layer is selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), antimony (Sb), and alloys thereof.
As shown in FIG. 6B, a gate-insulating layer 70 is formed on the array substrate 1 to cover the gate and common electrodes 52 and 54a, and on the gate-insulating layer 70, an active layer 72 is formed over gate electrode 52. The gate-insulating layer 70 is silicon nitride (SiNx) or silicon oxide (SiO2), while the active layer 72 includes an amorphous silicon layer (not shown) and a doped amorphous silicon layer (not shown).
As shown in FIG. 6C, a second metal layer is deposited and patterned to form the source and drain electrodes 62 and 64 on the active layer 72 and the pixel electrodes 66a on the gate-insulating layer 70. The pixel electrodes 66a are spaced apart from the adjacent common electrode 54a by a distance xe2x80x9cLxe2x80x9d.
As shown in FIG. 6D, a passivation layer 74 is formed to cover the source, drain, and pixel electrodes 62, 64, and 66a. The passivation layer 74 serves to protect the source, drain, and pixel electrodes 62, 64, and 66a from exterior humidity or contaminants.
As described above, the common and pixel electrodes 54a and 66a of the IPS LCD device are arranged in the same plane such that an in-plane electric field is applied parallel with the substrate 1. Though the IPS LCD device has an advantage of wide viewing angle, the aperture ratio and luminance of the IPS LCD panel are much lower than that of the LTN or STN LCD device. The common and pixel electrodes of the IPS LCD device are disposed on the same plane and are made from opaque metals. Therefore, large portions of light are reflected by the opaque metal, which causes the low aperture ratio. In addition, since the low aperture ratio results in a low brightness quality of the liquid crystal display device, incident light from a back light unit (not shown) must be brighter than in other devices to compensate for the low brightness quality of the IPS LCD device. Therefore, power consumption of the IPS LCD device increases.
In the U.S. Pat. No. 5,946,060, Nishiki, et al. teach a combined LCD device having advantages of the conventional TN LCD device and IPS LCD device. FIG. 7 shows the above-mentioned combined LCD device. A data line 60 and a first electrode 80 are disposed on a lower substrate 1a with a first interval between the data line 60 and the first electrode 80. An insulating layer 72 covers the data line 60 and the first electrode 80 on the lower substrate 1a. On the insulating layer 72, a second electrode 84 and a third electrode 86 are disposed with a second interval therebetween. Whereas, a counter electrode 90 is disposed on an upper substrate 1b such that the counter electrode 90 opposes the second and third electrodes 84 and 86 of the lower substrate 1a. The lower and upper substrates 1a and 1b oppose with each other, and a liquid crystal layer 10 is interposed therebetween.
The above-mentioned combined LCD device selectively adopts a first electric field xe2x80x9cE1xe2x80x9d and a second electric field xe2x80x9cE2xe2x80x9d, which are different from each other. The first electric field xe2x80x9cE1xe2x80x9d is applied perpendicular to the lower and upper substrates 1a and 1b, whereas the second electric field xe2x80x9cE2xe2x80x9d is applied parallel thereto. When the first electric field xe2x80x9cE1xe2x80x9d is applied between the counter electrode 90 and the first electrode 80, molecules of the liquid crystal layer 10 are aligned perpendicular to the lower and upper substrates 1a and 1b. Therefore, a first liquid crystal portion 10a is present between the lower and upper substrates 1a and 1b. On the contrary, when the second electric field xe2x80x9cE2xe2x80x9d is applied between the second electrode 84 and the third electrode 86, molecules of the liquid crystal layer 10 are aligned parallel to the lower and upper substrates 1a and 1b. In that case, a second liquid crystal portion 10b is present between the lower and upper substrates 1a and 1b. The first liquid crystal portion 10a corresponds to the TN mode shown in FIG. 1, whereas the second liquid crystal portion 10b corresponds to the IPS mode shown in FIG. 2. That is to say, the combined LCD device of FIG. 7 serves as the TN LCD device as well as the IPS LCD device depending on user""s desired use.
However, compared with the conventional IPS LCD device and TN LCD device, the above-mentioned combined LCD device needs additional elements and fabricating process. In addition, though the TN mode by the first liquid crystal portion 10a selectively substitutes for the IPS mode, the IPS mode by the second liquid crystal portion 10b yet has the problems of the conventional IPS LCD device.
Accordingly, the present invention is directed to an IPS LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an IPS LCD device having a high aperture ratio, a high brightness quality, and therefore a low power consumption.
Another object of the present invention is to provide an IPS LCD device having an organic passivation layer such that cross talk is prevented between a data line and a common electrode.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve the above object, the first preferred embodiment of the present invention provides an array substrate for an IPS LCD device. The IPS LCD device includes: first and second substrates; a gate line and a common line on the first substrate; a data line perpendicular to the gate line; a thin film transistor at a crossing portion between the gate and data lines, the thin film transistor having gate, source, and drain electrodes; a first insulating layer on the gate line; a second insulating layer on the first substrate, the second insulating layer having at least one contact hole; a plurality of transparent common electrodes on the second insulating layer; a plurality of transparent pixel electrodes on the second insulating layer; a transparent auxiliary pixel electrode on the second insulating layer and is electrically connected with the drain electrode via the contact hole; and a liquid crystal layer between the first and second substrates.
The IPS LCD device further includes a capacitor electrode on the first insulating layer. The capacitor electrode is preferably the same material as the source and drain electrodes, and one end of the pixel electrode preferably overlaps the capacitor electrode.
The IPS LCD device further includes a transparent auxiliary common electrode on the second insulating layer. The auxiliary common electrode preferably overlaps the common line and is electrically connected with the common line via the contact hole.
In the IPS LCD device, an outermost common electrode adjacent to the data line preferably overlaps a portion of the data line. The common and pixel electrodes are preferably selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO).
The IPS LCD device further includes a black matrix on the same layer as the gate line.
The second insulating layer is preferably selected from a group consisting of silicon oxide (SiOx) and silicon nitride (SiNx).
In another aspect, the second insulating layer is an organic material. The organic material is preferably selected from a group consisting of benzocyclobutene (BCB) and acryl resin.
In another aspect, the present invention provides a method of fabricating an IPS LCD device. The method includes: depositing and patterning a first metal on a substrate to form a common line and a gate line, the gate line including a gate electrode; forming a first insulating layer to cover the first metal; forming an active layer on the first insulating layer; depositing and patterning a second metal on the first insulating layer to form a data line, a source electrode, and a drain electrode, the data line being perpendicular to the gate line; forming a second insulating layer on an overall surface of the substrate to cover the second metal and the active layer, the second insulating layer having at least contact hole; and depositing and patterning a transparent conductive material on the second insulating layer such that a plurality of common electrodes, an auxiliary pixel electrode, and a plurality of pixel electrodes are formed on the same plane, wherein the auxiliary pixel electrode is electrically connected with the drain electrode via the contact hole.
A capacitor electrode is further formed on the first insulating layer in the step of patterning the first metal, and one end of the pixel electrode preferably overlaps the capacitor electrode.
An auxiliary common electrode is further formed on the second insulating layer in the step of patterning the transparent conductive material. The auxiliary common electrode preferably overlaps the common line and is electrically connected with the common line via the contact hole.
An outermost common electrode adjacent to the data line overlaps a portion of the data line.
The common and pixel electrodes are selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO).
The method further includes a steps of forming a black matrix on the same layer as the gate line such.
The second insulating layer is preferably selected from a group consisting of silicon dioxide (SiO2) and silicon nitride (SiNx).
In another aspect, the second insulating layer is an organic material. The organic material is preferably selected from a group consisting of benzocyclobutene (BCB) and acryl resin.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.